N. A. Krivova, I. Ermolli

Climate studies require long records of solar irradiance variations. Since direct irradiance measurements cover only limited time span, reconstructions of past irradiance variations are crucial. Such models require information on the evolution of solar surface magnetic field in the past, while information on facular regions prior to the period of direct irradiance measurements is scarce. As a result, reconstructions of past irradiance changes rely mainly on sunspot data, which requires certain assumptions about the relationship between sunspots and faculae. This has led to a discrepancy in estimates of long-term change in the solar irradiance. This issue can be resolved by utilising Ca II K observations, which is a largely unexplored dataset for irradiance reconstructions at the moment. We will discuss the potential of these data focusing in particular on the relationship between sunspots and Ca II K plage areas derived from a number of different Ca II K archives.